AVR: target/121794 - Invoke zero_reg less.

There are some cases where involing zero_reg is not needed and
where there are other sequences with the same efficiency.
An example is to use SBCI R,0 instead of SBC R,__zero_reg__
when R >= R16.  This may turn out to be better for small ISRs.

	PR target/121794
gcc/
	* config/avr/avr.cc (avr_out_compare): Only use zero_reg
	when there is no other sequence of the same length.
	(avr_out_plus_ext): Same.
	(avr_out_plus_1): Same.

(cherry picked from commit e3d14112b266479da9f8fd9c304e0ae15bf0ad25)